1. Field of the Invention
The present invention relates to techniques of a movable body apparatus including at least a movable body which is reciprocally and rotatably supported. More particularly, the present invention relates to a movable body apparatus, such as a resonance-type movable body apparatus, and an optical deflector using this. This optical deflector can be preferably used in optical instruments, such as image forming apparatuses like a scanning-type display, a laser beam printer and a digital copying machine.
2. Description of the Related Art
In recent years, an optical deflector for deflecting and scanning a light beam is used in optical disc apparatuses, laser beam printers and the like. Further, there have been proposed optical deflectors with a minute mirror produced using micro-machining techniques and capable of vibration in a resonance manner.
Such a resonance-type optical deflector has advantages as follows. Compared with an optical deflector using a rotary polygonal mirror, the size can be greatly reduced. The consumption electrical power also can be reduced. There theoretically exists no problem of so-called face tangle of a reflecting surface. In particular, with such an optical deflector formed of a Si single crystal capable of being fabricated by a semiconductor processing method, no metal fatigue exists theoretically, and the endurance property is typically excellent.
However, in resonance-type optical deflectors, the deflection angle (angular displacement) of a mirror changes in a sine-wave fashion. Accordingly, the angular velocity of the mirror varies. Japanese Patent Application Laid-Open No. 2005-208578 A (first Japanese reference, corresponding to U.S. Pat. Nos. 7,271,943 and 7,388,702, and US2008204843) discloses a method for correcting such property of a varying angular velocity. The deflection angle of a mirror has a predetermined relationship with the scan angle of a light beam deflected by the mirror, and hence these angles can be equivalently used. In this specification, the deflection angle (angular displacement) and the scan angle are used as a term having the same meaning.
The first Japanese reference discloses a micro-movable body apparatus in which a vibratory system with plural torsion springs and plural movable bodies has plural discrete characteristic vibratory modes. In this micro-movable body apparatus, plural discrete characteristic vibratory modes include a fundamental vibratory mode with a fundamental frequency and an even number-fold vibratory mode with a frequency equal to an approximately even number-fold of the fundamental frequency.
In this micro-movable body apparatus, a saw-tooth wave drive with an approximately equi-angular velocity range is achieved by vibrating the micro-movable body in those vibratory modes. The saw-tooth wave drive is illustrated in FIG. 22. In this drive, an angular displacement time of one way in a round trip motion of the movable body differs from that of the other way in the round trip motion within each period of the angular displacement. When the light beam deflected by the minute mirror under vibration of the saw-tooth wave drive is corrected by a correcting system or the like, approximately equi-velocity of a light spot formed on a scan surface can be attained without any change in the diameter of the light spot.
Meanwhile, the resonance-type optical deflector has the property that the resonance frequency changes due to a change in the ambient condition such as temperature. Japanese Patent Application Laid-Open No. 1995 (Heisei 7)-181415 A (second Japanese reference) discloses techniques of self-excited vibration as follows. An output signal of a detector for detecting the vibration of a movable body is fed back to a vibration input portion to control the driving frequency of the movable body. Thus, the movable body is always vibrated at its resonance frequency in response to a change in temperature.
When techniques of the second Japanese reference are applied to a system of plural torsion springs and plural movable bodies as disclosed in the first Japanese reference, the following disadvantage occurs. In techniques of the second Japanese reference, a delay phase difference between a target drive signal and the vibration of the torsion spring has a fixed value. In a case where plural factors exist for the phase delay between the drive signal and the vibration of the torsion spring, it is not easy to obtain an accurate delay phase difference.
Further, in techniques of the second Japanese reference, driving is performed in a single vibratory mode (bending deformation mode or torsional deformation mode). Therefore, when the driving of plural torsion springs is performed at a resonance frequency in the same kinds of plural vibratory modes about a common axis, the following disadvantage occurs.
For example, a vibratory system 200 as illustrated in FIG. 2 includes movable bodies 201 and 202, a torsion spring 211 for coupling these movable bodies, and a torsion spring 212 for coupling the movable body 202 to a support portion 221. In order that the movable body is driven as illustrated in FIG. 22, the driving needs to be performed by a combined wave of a fundamental driving wave in a fundamental vibratory mode at a frequency near a resonance frequency and an integer-fold driving wave at a double frequency, as illustrated in FIG. 19. When two movable bodies are driven by the combined wave having two frequency components in the same kinds of vibratory modes, it is not easy to control two frequency components of the angular displacement unless phases of frequency components of the drive signal are controlled.
Furthermore, in techniques of the second Japanese reference, it is not easy to drive the movable body at a frequency intentionally deviated from the resonance frequency.